Bonding orthodontic appliances (e.g., brackets) onto a patient's teeth requires an adhesive. Light-curable adhesives are most popular due to their single-component nature (i.e., no mixing). As will be appreciated, it is important to bond brackets or other attachments with a high bonding integrity as that translates to the durability and longevity of the bond. Two significant factors in attaining high bonding integrity are long working time and a quick, high-strength cure.
Long working time is essential to accommodate typical clinical dynamics. For example, for direct bonding of orthodontic brackets, there must be enough time to apply the adhesive paste to the bracket base under normal office lighting, then under exam lighting place the bracket on the patient's tooth, seat the bracket and extrude the excess adhesive, clean the excess, re-position the bracket to the final location, and perform a final clean-up. This entire sequence normally takes no longer than 1 minute per bracket. However, as is common, a clinical assistant can do the initial bracket placements while the clinician is performing other duties. In that case, some brackets may remain under high intensity lighting for several minutes. When the clinician does get to final bracket positioning, there is a high probability that the adhesive on some brackets may be long past viable working time and at risk of tearing, void formation, or other polymerization interruption that would result in poor molecular weight growth leading to compromised bond strength. In the worst case, the bracket(s) is rigidly fixed such that final re-positioning is not possible, in which case the entire procedure must be re-started. Compromised bond strength may manifest itself in initial bond failure often before the patient even leaves the office. Or, the bond may hold under initial light-force loadings, but further along in treatment the patient may bite something that would normally not dislodge the bracket, or during wire change the higher forces may cause the bond to fail.
Short cure time is desirable to quickly complete the bonding procedure. Quickly setting the adhesive reduces the overall appointment time thereby benefiting the patient and the clinician. Once the clinician has every bracket or appliance arranged as desired, the adhesive material should be rapid setting, or curing, to acceptable strength levels. This prevents any disturbance or dislocation of the brackets or other appliances, which may then lead to compromised bond strength due to disruption of the adhesive setting/curing reaction. Typical cure times for commercially available adhesives using a dental curing light range from about 10 to 40 seconds.
Currently available light-curable orthodontic adhesives have limits on the working time to curing time ratio (W/C ratio) that is achievable. It is believed that one factor affecting W/C ratios for known orthodontic adhesives is the adhesive initiator system. Some currently available adhesives utilize binary-type (2-component) initiator systems consisting of a sensitizer and an electron donor. One example of such a product is Enlight, available from Ormco Corporation. Other products, such as Transbond XT, available from Unitek/3M Corporation, use a ternary (3-component) initiator system, including a sensitizer, an electron donor, and a third component, an iodonium salt. U.S. Pat. No. 5,545,676 discloses such formulations. Sybron Corporation, in its Sequence product, has utilized a ternary initiator system that includes a sensitizer, an electron donor, and a peroxide catalyst. This initiator system is disclosed in U.S. Pat. No. 5,362,769.
The W/C ratio of orthodontic adhesives is affected by both the duration of the working time (W) of the adhesive and the duration of the curing time (C) of the adhesive. As a point of reference, the W/C ratio for Enlight Adhesive has been determined to be 2. Efforts aimed at improving the speed of curing and the physical properties of hardened adhesives typically have been directed at the curing light source. Improvements to conventional tungsten-quartz halogen bulb filtered light have approached their limit, and use of higher intensity light, such as xenon plasma pulsing or arcing (PAC) lights, have achieved limited success. Also, more monochromatic sources like [Argon] laser, and LED-type curing lights have attempted to match the absorption wavelengths of photoinitiators.
Providing orthodontic adhesives with a greater W/C ratio than currently available products is one of the objects of the present invention. Another objective of the present invention is an improved initiator system. A further objective is to provide orthodontic adhesives with desirable physical properties after curing, including impact toughness.